Sulfur Stable Isotope Discrimination in Rice: A Sulfur Isotope Mass Balance Study

The use of sulfur (S) stable isotopes to study S metabolism in plants is still limited by the relatively small number of studies. It is generally accepted that less S stable isotope discrimination occurs during sulfate (SO₄ ^2-) uptake. However, S metabolism and allocation are expected to produce separations of S stable isotopes among the different plant S pools and organs. In this study, we measured the S isotope composition of the main S pools of rice plants grown under different SO₄ ^2- availabilities in appropriate closed and open hydroponic-plant systems. The main results indicate that fractionation against ³⁴S occurred during SO₄ ^2- uptake. Fractionation was dependent on the amount of residual SO₄ ^2- in the solution, showing a biphasic behavior related to the relative expression of two SO₄ ^2- transporter genes (OsSULTR1;1 and OsSULTR1;2) in the roots. S isotope separations among S pools and organs were also observed as the result of substantial S isotope fractionations and mixing effects occurring during SO₄ ^2- assimilation and plant S partitioning. Since the S stable isotope separations conserve the memory of the physiological and metabolic activities that determined them, we here underline the potential of the ³²S/³⁴S analysis for the detailed characterization of the metabolic and molecular processes involved in plant S nutrition and homeostasis.